Green potatoes may not be as toxic as we thought

Beware of green potatoes, and peel every trace of green away: that's been standard advice for decades, and for good reason. When potatoes are exposed to light, these underground tubers interpret it as a sign that they're no longer completely buried in the soil. So they produce chlorophyll pigments to help them make use of the light's energy, and they produce bitter toxins to discourage animals from eating them. The toxins, alkaloids called solanine and chaconine, are about as powerful as their better-known cousin strychnine. They apparently interfere with the structure of all our cell membranes and also with the processing of a nerve transmitter (they inhibit acetylcholinesterase), which can cause hallucinations and convulsions. Because the color change in a potato parallels its accumulation of alkaloids, greenness is used as an indicator of toxicity and therefore irreversible spoilage. It's estimated that around 15% of the US potato crop is discarded on account of greening. But until recently, there has been little careful study of the toxin levels found in typical American potato varieties exposed to the light levels in typical markets.

N. Richard Knowles and colleagues at Washington State University exposed four different potato varieties (White Rose, Yukon Gold, Dark Red Norland, Russet Norkotah) to simulated store lighting for 10 days. They found that the correlation between color and alkaloid content was variable, with greening sometimes outpacing alkaloid accumulation. The highest alkaloid contents were found in the skins, where they sometimes exceeded the recommended maximum, an important fact for producers of potato-skin products. However, none of the varieties developed dangerous levels in the flesh.

These findings suggest that most greened potatoes need not be discarded. But the authors make two cautionary points. Potato alkaloids at low levels may have subtle toxic effects of which we're not yet aware. And because these alkaloids remain in the bloodstream for longer than 24 hours, people who eat potatoes every day may gradually accumulate toxic levels.

So it looks as though the occasional green potato is fine, but it's still not a good idea to buy them by the bagful.